DETONATION TRANSFER OF INSENSITIVE HIGH EXPLOSIVE ASSEMBLIES THROUGH A BULGED CLOSURE DISC

D. L. Jackson*

Lockheed Martin Space Systems Co. Denver, CO 80201-0179

W. W. Thorup

ATK Thiokol , Inc. Magna, UT 84044-0098

Lien C. Yang**

Northrop Grumman Mission Systems Missile Defense Division San Bernardino, CA 92402-1310

Greg Dougherty Goodrich-UPCO

Fairfield, CA 94533-0659

K. S. Orme***, Captain USAF, Space and Missile Systems Center

El Segundo, CA 90245-4683

* Member, AIAA ** Senior Member, AIAA *** Project Officer, Titan IV Program

ABSTRACT

Extensive investigation of a very rare bulged closure disc phenomenon was performed on the Titan IV Solid Rocket Motor Upgrade, SRMU, Linear Shaped Charge Assembly (LSCA) following a Lot Acceptance Test anomaly. The steel disc, spanning over a Hexanitrostilbene (HNS) load in a Transfer Charge, was approximately 0.101-mm (0.004-in.) thick and the bulge was on the order of 0.635-mm (0.025-in.) high with a diameter of 5.79 mm (0.228 inch). The charge provides the interface between an HNS loaded end-tip of a confined detonating cord and the linear-shaped charge. The bulge developed in three lot acceptance test samples, during dynamic testing. Possible causes of this anomaly were identified. A theoretical analysis and a

thorough testing series were performed to evaluate the effect of the condition on the detonation transfer.

Relevant parameters tested included the degree of the bulge, the gap width between the disc and the end-tip; the HNS powder condition inside the bulge, and the direction of propagation. No impact on the detonation transfer or the overall performance of the assembly was observed.

INTRODUCTION

To increase explosive interface protection and handling safety, and to achieve a hermetic seal, it is a common practice to weld a thin metal closure disc at the input/output ends of explosive components and devices. For a high explosive train, the disc is usually on the order of 0.127-mm (0.005-in.) thick to minimize possible transfer transients due to its presence. Thus, when the donor detonates, its closure disc accelerates to a high velocity that produces a high shock in the acceptor explosive upon impact. The latter is also sealed with a closure disc (or cup).

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40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit11 - 14 July 2004, Fort Lauderdale, Florida

AIAA 2004-3591

Copyright © 2004 by Lockheed Martin, ATK Thiokol, Northrop Grumman and Goodrich/UPCO. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

In reality, the transfer process is complicated. There must be an adequate gap between the explosives to allow the disc to accelerate. As the air sealed in the interface is compressed, it in addition to the drag effect, can slow down the disc. Fragmentation of the disc into multiple, high velocity particulates has also been reported. Since the entire process is not fully understood, government specifications require elaborate testing to demonstrate detonation transfer reliability at maximum and minimum gap widths with margins, and variable lateral and axial alignments. The discs also must undergo stringent inspections. Any significant indentations or scratches usually cause the unit to be rejected.

Recently, during post environments radiographic inspection of lot acceptance test samples of the Titan IVB Solid Rocket Motor Upgrade (SRMU) destruct charge, the Linear Shaped Charge Assembly (LSCA), revealed that the transfer charge in several units exhibited a very rare “bulged closure disc” condition. The bulges were on the outboard surface, with a bubble-shaped protrusion on the order of 0.038 to 0.635-mm (0.007 to 0.025-in.). These heights are comparable to the nominal gap between the Reinforced Confined Detonating Cord (RCDC) end-tip and the transfer charge (Figure 1).

Figure 1. Presence of the Bulged Closure Disc in the

Explosive Train

The consensus was that this unusual condition might cause a detonation transfer failure because the donor disc ejected from the RCDC end-tip would initially impact the bulged transfer charge disc and not promptly involve the acceptor explosive underneath it.

The root cause of the phenomenon was not conclusively identified, but strong indications pointed to the mechanical disturbance induced by the dynamic environmental testing. Investigation has shown that the explosive underneath the bulged disc was not degraded and had not experienced chemical changes. A theoretical analysis on selected cases of different configurations indicated that this condition would not compromise transfer reliability. In addition, a detailed

review of archive data revealed this anomaly had occurred on two units during lot acceptance test of two earlier LSCA lots. They were successfully fired in the function tests. In response to these findings, a detailed testing series was developed to evaluate this phenomenon. For the test, artificially press-formed bulge discs were used to complete the transfer charge fabrication. Fifty-five donor/acceptor assemblies were fabricated following a test matrix that varied four relevant parameters:

• Degree of disc bulge • Gap between donor and acceptor charges • Presence and absence of loose HNS powder

underneath the bulge • Direction of the detonation propagation

All samples produced full detonation transfer. This result verified the analytic predication that a bulged disc does not impact hardware performance. Therefore, LSCA lots exhibiting the bulged disc anomaly in lot acceptance tests can be reliably used for flight.

This paper provides a detailed report on the investigation, analysis, testing design/implementation, and test results. To facilitate the presentation, the design and testing methodology of the LSCA pertinent to the bulged disc are briefly summarized in the following two sections.

SRMU LSCA DESIGN

LSCA have been used for solid rocket motor destruct since the late 1950’s. Successful examples include the flight termination subsystems designed for the U.S. Air Force’s Minuteman and Peacekeeper Intercontinental Ballistic Missile Systems (References 1 through 3). The uniqueness of the design of the SRMU LSCA was driven by the fact that the SRMU is a very large motor.

Figure 2. LSCA/RCDC Deployment Configuration on

SRMU

As shown in Figure 2, seven LSCAs, interconnected by RCDC lines installed in the motor raceway, are used to cover the full length of the motor and facilitate

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installation logistics. Each LSCA is approximately 214-cm (84.3-in.) long. For redundancy, an RCDC assembly consisting of three interconnected RCDC segments is used as an alternate propagation route. The general features of the LSCA are shown in Figure 3.

Figure 3. Exterior, Interior, and Installation

Configuration of LSCA

In order for a LSCA to reliably sever the ~2.54-cm (1.0-in.) thick SRMU graphite composite case with a ~25% margin, a relatively large Homocyclonit (HMX) explosive loading of ~1.59 g/cm (~750 gr./ft) was designed for the aluminum sheath based LSC. The chevron-shaped LSC cross section is ~1.57-cm (~0.62-in.) tall and ~1.88-cm (~0.74-in.) wide. To achieve a better interface with the well-established RCDC end-tip, which has a diameter of ~0.38-cm (~0.15-in.), a transfer charge containing pressed HNS I explosive (Reference 4), in a truncated conic configuration was designed. The small end, 7.1-mm (0.280-in.) in diameter, faces the RCDC end-tip; the larger end, 1.4-cm (0.550-in.) in diameter, is integrated with the LSC end by a thin layer of C-7 epoxy (Figure 4).

Figure 4. Interior Construction of LSCA Sliding End-

Fitting (Same for Both End Interiors)

The epoxy thickness is typically ~0.13-mm (0.005-in.) thick, with a maximum allowed thickness of 0.76-mm (0.030-in.). Both HNS end surfaces are protected by a

0.101-mm (0.004-in.) thick steel closure disc welded on the end face of the transfer charge. The LSC ends are also welded with a ~0.127-mm (0.005-in.) thick aluminum foil closure. The LSC/transfer charge assembly was in turn integrated in a hollow cylindrical end-fitting and secured by potting compound and four setscrews.

The adoption of a conic charge configuration made it difficult to implement the traditional practice of loading in multiple increments, as that used for a straight long bore. Therefore, a short cone was designed so that 80% of HNS was consolidated in a single increment, with the balance loaded afterwards. The density of the charge is ~1.35 g/cm3. This approach provided better control of the flush of the charge surface respective to the charge holder end surface, but the holding force on the powder matrix by the charge cavity wall may not be as strong as in the case of a long, straight charge column.

To maximize motor case severing effectiveness, the LSC is held at a ~2.86-cm (~1.125-in.) standoff height above the case. Thus, each LSCA in the raceway is supported in a bridge-like configuration (Figure 3). The downward end is rigidly fastened on a bracket in the raceway by screws. Its upper end is a sliding end where the cylindrical LSCA end-fitting is allowed to slide in a cylindrical hollow housing buffered by a rubber O-ring. Six sliding brackets fastened on their respective raceway brackets provide additional radial and lateral support. The intent of this design is to offset axial motor case expansion during motor action. This point support introduces significant axial response amplifications on the LSCA when it is under dynamic environments.

Further information on the LSCA and RCDC design and function/transfer behavior is contained in previous reports (References 5 through 9).

TESTING METHODOLOGY

Several challenges in LSCA testing, together with their resolutions, are described in this section. They are potentially relevant in creating the bulged disc anomaly.

First, due to the limitation of the dynamic testing equipment, the tests are commonly conducted on subscale samples. Subscale LSCA samples 92.1-cm (36.25-in.) in length were used for both qualification and lot acceptance testing. These samples were prepared by cutting off the already completed full-scale LSCAs to the length required and re-terminating the severed LSC ends with new LSCA ends (at either the fixed or sliding end). Because the mass of the deleted LSC, on the order of 0.5-kg (1.1-lb) was significant, a short metal cylinder of this mass was installed via the RCDC port at the sliding end during longitudinal

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dynamic testing. Its main purpose was to simulate the impact of the mass in the response along the LSCA longitudinal axis.

Figure 5. Design and Assembly of the Mass Simulator

As shown in Figure 5, the cylinder was secured by a U-clamp and spacer plate which were fastened to the test fixture. There were adequate clearances, on the order of 0.1-mm (0.005-in.), in this restraint mechanism to allow free longitudinal movement of the LSCA in the sliding end-fitting but restrain movements in lateral directions. The mass simulator was used for both the shock test and the longitudinal vibration tests; however, the U-bolt restraint was only used in the shock test. Figure 6 shows the shock test configuration. The subscale LSCA units were secured on the shock test platform with the weight simulators installed on the sliding end-fittings. Note also that only two of the six LSC supporting brackets were used for the subscale units.

Figure 6. Test Configuration of Subscale LSCA Units

on Shock Test Plate Fixture

Second, a 2.23-kN (500-lb), 3.5 minute pull test between the two LSCA end-fittings was adopted. This force is beyond the capability of the epoxy and rubber potting used in fabricating the end-fittings. LSCs separated from their end-fitting housing during design

development. To enable the design to withstand the tensile load, four setscrews were added in the housing wall and driven into the LSC surface. During the pull test, some relaxation of the setscrew footings in the LSC is possible. This may result in a high dynamic response in the end-fitting during dynamic environments which may contribute to the formation of the bulged disc.

Third, although a high shock environment is not anticipated in normal SRMU flight operation, Range Safety regulation EWR-127-1 (Reference 10) requires a minimum level shock test of 1,300g from 2,000 to 10,000Hz. The intent of the shock is to simulate the worst-case disturbance that may result from an unexpected vehicle breakup. This level cannot be achieved by a precision mechanical shock generator or a vibration shaker. Instead, a shock was generated by detonating explosives (commonly called pyro-shock test). This testing technique is known for difficulty in level control, and over-tests in portions of the frequency domains usually occur. In addition, the real high shock response spikes can exceed 10,000Hz in frequency and induce severe damage, e.g., cracks and debonding in the epoxy composition and interfaces. As will be discussed, epoxy debonding between the transfer charge and LSC contributed to the formation of the bulged disc.

Fourth, the local vibration levels at the LSCA end-fittings are high. The raceway flight vibration level applicable to LSCA, 20.1 grms, is very moderate. However, EWR-127-1 requires a minimum test margin of +6 dB; i.e., the test must be conducted at 40.1 grms. (Unlike the shock test, the vibration test was highly controllable. A typical raceway vibration power spectral density (PSD) is shown in Figure 7.)

Figure 7. Raceway Vibration Power Spectral density,

Measured Value vs. Limits

Further, triple length test duration over the expected duration is required for the qualification and acceptance tests. Resonation of the sliding LSCA end amplified the input level to high levels ranging from 110 to 130 grms,

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as measured on the LSCA end-fittings. For this reason, in the RCDC lot acceptance test, eight RCDC lines were required to be installed on an inert LSCA and subjected to this high level vibration due to amplification. As will be discussed, the high level vibration could contribute to the formation of the bulged disc.

The eight cycles of the temperature cycling test (-29o F and +175o F) is considered a moderate test. Prior to the environment test, X-ray inspection was performed at assembly level. N-ray inspection was performed on the transfer charge at the subassembly level. Because the HNS N-ray image was obscured by the steel transfer charge holder, very limited information was useful. Therefore, after the environment testing, only X-ray inspection was performed. The bulged disc anomaly was observed during this inspection.

DESCRIPTION OF THE ANOMALY

The bulged disc condition was first discovered in the routine post environment X-ray inspection in LSCA Lot-7 lot acceptance test. S/N 95, one of the nine subscale units tested, exhibited a bulge in the transfer charge closure disc in the sliding end of the unit. The height of the bulge was approximately 0.64-mm (0.025-in.). The lot was scheduled for 58 deliverable LSCA units. Nine units were tested per EWR-127-1 (10% of the lot or nine units, whichever is greater). Figure 8 shows the original X-ray image of the region of interest. Figure 9 is a magnified image zoomed in on the bulged disc, and Figure 10 is a digital enhanced image of the region. The X-ray record from the pre-environment test inspection did not show this anomalous condition.

Figure 8. Post Environmental Test X-ray Record of

Lot 7 (S/N 95) Sliding End Showing the Bulged Closure Disc on the Transfer Charge

Figure 9. Close-up View of the Bulged Disc in X-ray

Record (S/N 95)

Figure 10. Digital Enhanced X-ray Image of the Bulged

Disc (S/N 95)

Reexamination of post environment X-ray records for acceptance samples in all six previous LSCA lots uncovered two more anomalies that are similar. The sliding end of one unit in Lot 5 showed a 0.58-mm (0.023-in.) bulge, and the fixed end of one unit in Lot 4 showed a 0.18-mm (0.007-in.) bulge. Both were in the outboard closure disc. Samples from Lots 1 through 3 did not exhibit this condition. The database also showed that a nominal 0.025 to 0.051-mm (0.001 to 0.002-in.) bulge is normal because of a designed HNS loading over-flush condition . All three anomalous LSCA ends were made during the fabrication of the subscale unit; i.e., they were not the original ends of the full-scale units. As will be discussed, this fact could have contributed to the formation of the anomaly.

During the entire LSCA program, one hundred and thirty (130) subscale LSCAs have been fabricated, with 260 LSCA ends tested in the six lot acceptance tests. Therefore, the observed three anomalous units represented a rather rare condition. Other than film records, the Lot 4 and Lot 5 anomalous units were no longer available for investigation, as they had been successfully function tested. The test success criteria include acceptable LSC Velocity of Detonation and jet penetration into a witness plate, and successful detonation transfer into and out of the LSCA.

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INITIAL INVESTIGATION

Due to the discovery of the bulge, a detailed “fault tree” was formulated. It consisted of over eighty elements in the six categories of possible faults in manufacturing, parts/material, design, test, handling and personnel. They were focused on supporting two apparent possible top-level failure scenarios: the bulge was caused by gas pressures created in the HNS or by mechanical forces created by testing.

Initially, the reaction of HNS in the transfer charge was considered a real possibility, as debris having the appearance of soot was observed in the RCDC port in the anomalous LSCA end. However, a gas-chromatogram/mass spectrum analysis of the soot failed to show typical products of HNS reaction: N-butyl-Benzenesulfonamide and Tetramethybutyl Phenol were not present. (The soot was found mainly to be fine aluminum particulates). In addition, impact testing of HNS powder at a level much higher than the shock testing could not induce any notable HNS reaction. (The powder at times changed to a translucent appearance, but no black soot was observed.) Hydrostatic pressurization testing of the bare closure disc in LSCA end configuration did produce a 0.66-mm (0.026-in.) bulge at a hydraulic pressure of ~12.2-MP (1800-psi), which was consistent with the calculated value of 8.2-MP (1200-psi). However, the bulge was spherical, which differed from the flat top bulge observed in S/N 95. It was also concluded that, at the temperature of testing, the volatile contaminates in HNS, if present, (e.g., moisture and acetone), would not have a sufficiently high vapor pressure to produce the disc bulge.

Laboratory tests showed that by pressing the transfer charge from the inner end with a 9.5-mm (0.375-in.) diameter ram, a disc bulge of 0.64-mm (0.025-in.) could be formed under a force of 8.02-kN (1800-lbf). This result was not directly applicable to the bulge disc phenomenon, as it corresponded to 110.9-MP (16,300-psi) of pressure, a level much higher than what could be induced at the LSC and transfer charge interface by the dynamic testing. In addition, the small diameter effect of the ram is not representative for the LSC, which is larger than the HNS inner diameter. One additional laboratory tests did enhance the credibility of the mechanical scenario. An empty transfer charge housing had a closure disc welded onto it and then was inserted into an LSCA end fitting housing. It was then struck with a hammer about 200 times which peened the disc between the transfer charge housing and the end fitting housing and flowed the disc material into the end fitting port. This created a bulged disc nearly identical to that observed in S/N 95. This supports the idea that if the transfer charge is loose to rattle around during dynamic

testing, as evidenced by aluminum particulates in the end fitting RCDC port of SN 95 and by damage to the transfer charge and end fitting as documented below, impact of the transfer charge on the end fitting may produce the bulge. This was verified for the SN 95 disc which, when sectioned and measured for thickness, showed a thinning of the disc in the area of impact but no thinning in the area of the bulge.

Thorough reviews of extensive manufacturing and testing records ruled out the majority of the fault tree elements as causes of the anomaly. Two outstanding discrepancies were uncovered. Both were related to the new personnel in test execution and both may have contributed to the anomaly. First, it was confirmed that the type, amount and configuration of the explosive used in the shock test for the recent three LSCA lots were different from that used in testing the first three LSCA lots. The total explosive mass increased approximately twofold. A typical shock response spectrum for Lot 7 shock testing is shown in Figure 11.

Figure 11. Shock Response Spectrum Record for

S/N 95 While the over-test in the high frequency regime, from 2,000 to 10,000-Hz, was common in the shock test, the positive excursion in the low frequency regime, from 600 to 2,000-Hz, was unusual and not present in the earlier tests.

Second, the spacer plate on the mass simulator was installed incorrectly. The stop nuts for the plate were not used and the restraining nuts were over-tightened by a quarter turn beyond the intended snug condition. This led to a hard clamping on the simulator, as can be seen in Figure 12.

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Figure 12. Mass Simulator Assembly Configuration

Used for S/N 95 (Recreated for Clarity)

The plate was visibly bent. This condition impeded the movement of the simulator and, in turn, the sliding LSCA end. Finite element analysis indicated that this condition could increase the stress level in the epoxy bonds inside the end-fitting and lead to possible bond breakage. Laboratory testing determined that, under steady pulling, the clamping action by the U-bolt created up to 401-N (90-lb) of equivalent loading force. It defeated the intent of both the sliding end design and the mass simulator.

An ad-hoc test was performed to evaluate the effect of clamping force on dynamic response in the LSCA sliding end region. Three LSCA subscale units were used: one without the mass simulator attachment, one with the mass simulator clamped in the intended manner so that the LSCA sliding end and the simulator were free to move in the longitudinal axis, and one with the mass simulator hard clamped, inhibiting movement in all three axes. Miniature accelerometers were attached on both LSCA ends and the mass simulator. Two strain gages were installed on the LSC, about 2.54-cm (1.0-in.) from each LSCA end-fitting. Acceptance shock and vibration tests were performed. The collected data did not show dramatic differences among the different configurations. The test was considered as inconclusive because the measurements were performed far away from the intended critical region near the LSC/transfer-charge interface and because of insufficient instrumentation resolution. The testing did not produce a bulge in any of the six closure discs.

S/N 95 DISASSEMBLY INSPECTION

Disassembly of the bulged unit began after all nondestructive observations had been completed. The sliding end of S/N 95 was machined off from the LSCA and the sliding housing removed. A through circumferential transverse cut on the housing along the

middle point of the transfer charge enabled the removal of the RCDC port portion of the housing. It was found that the transfer charge was loose and separated from the LSC end, which was imbedded in the rubber potting. Prior to this point, no indication of this result had been observed, either by X-ray or by rattling the transfer charge. A helium leak test indicated that the hermetic seal of the charge was intact. The set screws were found to be intact; i.e., neither loose threads nor degradation of screw/LSC contacts were observed.

Figures 13 through 20 document the condition of the components after the disassembly/dissection.

Figure 13. Outboard End Figure 14. Side View

of Transfer Charge after Disassembly (S/N 95)

Figure 15. Condition of Sliding End-fitting Interior

After Disassembly (S/N 95)

Figure 16. Condition of LSC End Showing Deposit of

Abraded Aluminum (Sliding End S/N 95)

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Figure 17. Alternate View of the Same LSC End

Showing Concentric Marks

Figure 18. Condition of Back end of Transfer Charge

(Sliding end S/N 95)

Figure 19. Condition of HNS Charge Under Bulged Disc Showing Missing Powder Along the Perimeter

Figure 20. Inner Side of the Bulged Disc Showing Fine HNS Powder Deposition and Translucent HNS Layer

Outside the Perimeter (Scratch Was Made by Disassembly Process)

For comparison, Figures 21 through 23 show the condition of selected components disassembled from a normal LSCA end.

Figure 21. Transfer Charge Disassembled From a

Normal LSCA Sliding End

Figure 22. Condition of a Normal Sliding End-fitting

Interior After Disassembly

Figure 23. Condition of LSC End of a Disassembled

Normal Sliding End

Based on this evidence, the following remarks can be made: • Darkened surfaces on the inner side of the transfer

charge and the LSC end indicate that the aluminum in the interface had suffered some degree of abrasion.

• The concentric markings on the LSC end surface indicate a probable rotation motion of the transfer charge relative to the fixed LSC end surface.

• Darkened marks on the transfer charge outer surface and the bottom surface of the end housing indicate possible abrasion of the aluminum surface.

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• Rugged ridge markings on the housing bottom surface appear to be an impact mark by the weld stitch contour on the transfer charge outer surface.

• HNS broke loose along the orifice of the outer surface, indicating the rim region was subjected to a high force, possibly over 68.0-MP (10,000-psi) of pressure, the consolidation pressure of HNS in the transfer charge.

• Thin layer coating of fine HNS on the interior surface of the bulged closure disc indicates that the region might have experienced vigorous vibration to the extent that the chunks of broken-off HNS from the rim region were futher dispersed into fine particles. A thin layer of translucent HNS was observed in the gap between the disc and the transfer charge end surface, between the weld stitches line and the charge rim, indicating that the powder present there was compressed by impact force like that observed in the laboratory impact test.

• No debond between the LSC and the transfer charge was observed in the normal LSCA endfittings.

ANOMALY ROOT CAUSE

The findings of this investigation point to the following plausible scenario.

The C-7/W epoxy bond between the transfer charge and the LSC end in S/N 95 was likely broken by the shock test. C-7/W epoxy is a hard but brittle material. Cracks in C-7/W potting were observed in many acceptance test units after the environment testing. Both shock over-test and improper installation of the mass simulator could have contributed to the crack growth toward the bond breakage. The fact that all three LSCA ends that exhibited the bulged disc anomaly were fabricated for the subscale units, points to this fabrication process. The fixture and manufacturing procedures were the same as for the full-scale LSCA ends. However, during the curing of the epoxy bond between the LSC and transfer charge and the potting of the entire end, the LSC was held vertically with the weight of the assembly imposed on the bond under curing. Therefore, the full-scale unit, being heavier than the subscale unit, potentially formed a more uniform and stronger bond. In this scenario, the subscale LSCA ends may exhibit considerable variability in the bond strength that may explain why only a limited number of bulged discs were observed.

Once the transfer charge has broken loose, it will respond to the vibration input as a separate item. The magnitude of the response would be much higher than the original already fairly high response, as dictated by the void space available. The resulting multiple rattling and pounding of the transfer charge onto the end-fitting

housing bottom could produce a bulge disc condition as has been assessed previously. It could also create enough friction to cause the observed aluminum abrasion.

It is believed that subjecting an LSCA subscale unit with a deliberately fabricated unbonded transfer charge to the lot acceptance dynamic testing would create a bulged disc condition. Most importantly, the investigation indicated that the bulge was likely caused by the design and execution of testing of the subscale units and was not necessarily related to the full-scale flight units. Examples of design and testing improvements that may be incorporated are: Eliminate the use of epoxy bond as the primary means for LSC/transfer-charge interface mechanical integrity; Consider using full-scale units for lot acceptance testing; and reduce the force used in the pulling test.

Because Lot 7 was the last LSCA production lot for the Titan IV Program, there are no plans to implement these potential improvements. Instead, the next two sections are dedicated to the theoretical assessment and testing program for characterizing the effect of a bulged disc in detonation transfer in an LSCA, if it indeed occurs during flight.

THEORETICAL ASSESSMENT OF IMPACT OF BULGED DISC

Earlier work (Reference 11) reported that the far-field (centimeters or inches away) products of a closure disc ejected from a detonating cord end-tip consist of multiple, irregular high energy/speed particulates. Thus, they are difficult to model. During recent work in the Titan IVB High Voltage Detonator, the speed of the closure was successfully measured by the Department of Energy pioneered Velocity Interferometer System for Any Reflector (VISAR) technique and was used to gage the detonator output in lieu of a steel witness plate (Reference 12). Therefore, in the near field, the closure acts like a flyer plate. Under a normal transfer configuration across a gap of ~0.51-mm (0.020-in.), as References 5 and 6 show by modeling, a 0.13-mm (0.005-in.) thick steel closure of a detonating cord end-tip (~100 mg of ~1.6 g/cm3 density HNS I) acquires a near terminal speed of ~1.5-mm/µs in a distance on the order of twice the disc thickness, ~0.25-mm (0.010-in.). Upon impacting on a steel closure of an acceptor charge, the end-tip closure generates a shock pressure on the order of ~340 kbars in the discs. However, if the acceptor charge is 1.6 g/cm3 density HNS, only ~73 kbars of shock pressure will be imparted into the HNS due to the impedance mismatch between steel and HNS. This is sufficient to instantaneously detonate HNS because its threshold for the instantaneous detonation initiation is ~73 kbars (Reference 13). The non-instantaneous initiation threshold shock for HNS is

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estimated to be ~ 41.5 kbars by scaling the HMX data (~77.6 and 44.1 kbars respectively, Reference 14).

The effect of a 0.64-mm (0.025-in.) bulge in the transfer charge closure disc was qualitatively assessed for large or small interface gap and the direction of the detonation transfer yielding the following four cases:

Case 1. Large gap, RCDC to transfer charge. The RCDC closure will puncture the bulged disc to form a flyer plate of twice the thickness but half the speed, 0.75-mm/µs (by knowing that the energy loss due to punctuation is negligible as compared to the kinetic energy.). When it impacts the HNS, it generates a shock pressure of 36.4 kbars in the HNS, per the impedance matching method and Hugoniot data of steel and 1.6 g/cm3 HNS from Reference 13. Figure 24 shows this calculation graphically, presented in shock pressure in kbars vs. particle velocity in mm/µs. The P-u relation of the steel, Curve A, is,

P = 129.76 u2 + 356.19 u (1)

Where P is the shock in the steel disc in kilobar units and u is the corresponding particle velocity in mm/µs. The mirror image of Curve A is formed from Curve A by passing through the point P = 0, u = 0.75 mm/µs. The intersection of this curve with the P-u relation of the 1.6 g/cm3 density HNS (linear approximation is used due to limitation of available data and 1.6 g/cm3 density is used for the ~1.35 g/cm3 HNS in the transfer charge as a worst case),

P ≅ 55.649 u (2)

yields 36.4 kbars of impact shock pressure in the HNS. This level of shock is below 41.5 kbars required to initiate the HNS by a 0.041 µs shock pulse corresponding to the closure thickness. However, since the new flyer thickness is doubled, the pulse duration and the stress energy are also doubled and the equivalent shock level is increased to 36.4 kbars x (2)0.5

= 51.5 kbars, which is above the HNS detonation initiation threshold.

Figure 24. Shock Pressure Imparted by a Flyer Plate

with 0.75-mm/µs Speed

Case 2. Large gap, transfer charge to RCDC. Because of the large mass of HNS presented in the transfer charge, the ejected bulged disc is accelerated to the near terminal velocity of ~1.5 mm/µs. The disc flyer initiated the RCDC end-tip in a manner identical to that in the normal transfer.

Case 3. Small gap, RCDC to transfer charge. It is envisioned that the two closure discs will contact each other early in the process to form a single disc 0.25-mm (0.010-in.) thick and be accelerated together to the terminal velocity of ~1.5-mm/µs. Upon impact, they will create a shock of 72.81-kbars in the HNS as shown in the impedance matching plot in Figure 25. The figure is similar to Figure 24, except the mirror image is formed from Curve A by passing through the point P = 0, u = 1.5 mm/µs, resulting in an impact shock pressure in the HNS of 72.81 kbars. The transfer reliability is actually enhanced because the longer shock duration due to the double disc thickness is equivalent to an effective level of 72.81 kbars x (2)0.5 = 103 kbars.

Figure 24. Shock Pressure Imparted by a Flyer Plate

with 1.5-mm/µs Speed

Case 4. Small gap, transfer charge to RCDC. At first glance, a gap smaller than 0.13-mm (0.010-in.) may not provide sufficient run distance for successful acceleration, or, in the worst case in which the two discs are initially in contact with each other, zero room for acceleration. However, as has been shown in the modeling in References 5 and 6, the gas generated by HNS detonation could impart a shock pressure of 120 to 230-kbars into the acceptor closure disc. Once again, due to the impedance mismatch, the pressure imparted into the HNS in the end-tip may be below the 41.5 kbars initiation threshold for 1.6 g/cm3 density HNS by a 0.041 µs shock pulse. The gas pressure of microseconds in duration, being over one order in magnitude longer than the impact pressure pulse by a closure disc, will achieve the HNS detonation initiation in according to the critical energy P2τ theory (Reference 13).

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The arguments in Cases 1 and 2 are applicable to a bulged disc with smaller heights, e.g., the 0.18-mm (0.007-in.) bulged unit in LSCA Lot 4. The argument in Case 4 is applicable to Cases 3 and 4, if bulged discs with the same small height are presented. The presence of loose HNS powder under the bulged disc is considered as a more favorable condition for the detonation transfer because it is well known that low density; fine particle-size insensitive high explosives have a lower detonation initiation threshold than the same explosive in a compacted high-density state. The forgoing discussion indicates that, from the analytic point of view, there is nothing out of the ordinary for detonation transfer in the presence of a bulged transfer charge closure disc in the LSCA.

TRANSFER RELIABILITY TEST VERIFICATION

The primary purpose of this test series was to demonstrate that the EWR-127-1 requirements on the gap between the transfer charge and the RCDC end-tip are met in the presence of the bulged disc. The criteria are: successful firing tests using a gap that is at least four times the maximum allowable design gap, that is the stack-up of drawing specified tolerances, which is 0.51 to 1.45-mm (0.020 to 0.057-in.), or a 3.81-mm (0.150-in.) gap, whichever is greater and a gap which is half the width of the minimum allowable. (For this reason the name “Margin Test” was adopted for the testing series.) The extreme values of the bulge height, the direction of the transfer, and the presence or absence of HNS loose powder under the bulge were also included in the test matrix. The formulation of the matrix proceeded as follows. For clarity, lengths in metric units are not listed. They can be converted by scaling factors of 2.54 cm/in. or 25.4 mm/in.

? MAX/MIN BULGE DEFINITION

The two units in Lots 5 and 7 had bulges of 0.023 to 0.025-in.; while the unit in Lot 4 had a smaller bulge of approximately 0.007-in.

Determination of max bulge size:

• RCDC standoff range: 0.020 to 0.057-in. (0.039-in. nominal).

• Assuming nominal standoff, max bulge size at interference with RCDC is 0.039-in.

The margin tests assumed a max bulge size of 0.035 to 0.045-in. This test gap provided a 60% margin over existing bulges and enveloped the worst-case bulge.

Determination of min bulge size:

• Actual minimum bulge heights seen after loading were approximately 0.001 to 0.002-in. and filled with

pressed HNS. Since this is common, it was desirable to use a greater bulge size.

The margin tests assumed a min bulge size of 0.004 to 0.009-in. This test gap provided some minimum standoff between the disc and HNS column and was in the range of the gap seen in Lot 4.

The bulge was created in the closure disc by a special press tool. Figure 26 shows top and side views of a typical disc with a large bulge. Bulge sizes were verified by dimensional inspection (both prior to, and after welding the disc to the transfer charge body) using a 0.0625-in. ball drop indicator as shown in Figure 27. Post-weld bulge heights were recorded in data sheets.

Figure 26. Two Views of an Artificially Manufactured

Bulged Closure Disc

Figure 27. Method Used to Measure the Height of the

Bulge

MARGIN TEST GAP DEFINITION

Margin test gaps typically tested per DOD-E-83578A (Reference 15)

4 x max design gap (additional 0.050-in. from DOD-E-83578A was not used; EWR-127-1 requirement was adopted) or 0.150-in. minimum (5 units to be tested)

0.5 x min design gap (5 units to be tested)

Standoff gap between RCDC and disc in LSCA with un-bulged closure has been calculated to be 0.020 to 0.057-in.

Bulge sizes defined above: 0.004 to 0.009-in. min; 0.035 to 0.045-in. max

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Gaps used with min bulge size:

Min gap: (0.020 minus 0.009) x 0.5 = 0.006-in. (In an attempt to ensure some small gap was introduced, gap range was set at 0.004 to 0.009-in.)

Max gap: (0.057 minus 0.004) x 4 = 0.212-in. minimum

Gaps used with max bulge size:

Min gap would be interference between RCDC and disc, but some small gap should be introduced as worst case; therefore, gap range was set at 0.004 to 0.009-in. Max gap: (0.057 minus 0.035) x 4 = 0.088-in. (Gap was set to default at 0.150-in. min.)

Gaps were determined using mechanical measurements taken before and during assembly (Dimensions A and B illustrated in Figures 28 and 29) and verified by X-ray inspection after assembly (Dimension C illustrated in Figure 30). Where the X-ray resolution was insufficient, as often occurred with small gaps and/or small bulges, additional elaborated mechanical measurements for the gap width were performed by measuring the depth of the disc from the top of the RCDC port and subtracting from it the SMDC end-tip length and the threaded port well depth. Because of the relatively large tolerance of ~0.015-in. allowed for the gap between the transfer charge and the end-fitting bottom, the precision end-tip to bulged disc gap control required the use of combinations of 0.002-in.-thick and 0.005-in.-thick brass shims as spacers at this interface.

Figure 28. Definition of Mechanical Measurement for

Dimension A

Figure 29. Definition of Mechanical Measurement for

Dimension B

Figure 30. Definition of X-ray Measurement for

Dimension C

BULGE FILL DEFINITION

S/N 95 transfer charge dissection revealed some loose HNS in the interface between the disc and the HNS column. Two bulge fill conditions were included for these margins tests:

Air between disc and HNS charge.

Transfer charges had a pressed flat HNS end surface condition at the small diameter prior to welding the bulged disc in place.

Loose HNS between disc and HNS column simulating what was observed in S/N 95.

Transfer charges were loaded per normal procedure.

HNS surface at the small diameter of the transfer charge was scraped with a pick or similar tool to break up the edges of the powder column and simulate the appearance of the HNS as seen in the S/N 95 (bulged) dissected charge (See Figure 19). This photo was used as a visual standard.

Loose powder created from breaking up the edges of the charge was swept up onto the center of the HNS column

Digital photographs were taken of at least one representative unit from each test group (as defined in Table 1). Each transfer charge was identified by a unit number or serial number; each photograph identified the number of the subject unit.

The bulged closure disc was then welded in place.

The condition of the bulge fill (i.e. air or loosened powder) was verified just prior to welding the disc in place.

The test matrix, specific sample conditions, and sample numbers are shown in Table 1. Note that not all possible combinations were included; e.g., small bulge and transfer from LSC to RCDC was not tested due to the robust explosive mass presented in the donor, as discussed previously. Since the primary control of the gap is the RCDC port length, four end-fitting types with different port lengths were fabricated to complete the test samples. LSCs ~14-in. long were used for fabrication of the 55 test samples (1-in. embedded in the end-fitting). Essential elements and procedures for

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full-scale and subscale LSCA fabrication were implemented for the test samples. Lockwires, O-rings, chemical conversion, and passivation process were omitted. The end bushing (with chevron hole) was not

used; therefore, the back end of the end-fitting was filled with epoxy, and additional setscrews were used to center the LSC in the fitting.

Table 1. Bulged Disc Transfer Margin Test Matrix

Test Group

Bulge (inches)

Bulge Fill (Empty or Powder

Scraped from Pressed HNS)

Gap / Standoff (inches)

Initiate From Quantity

1 .035 - .045 Powder .150 min SMDC 5 2 .035 - .045 Empty .150 min SMDC 5 3 .035 - .045 Powder .004 - .009 SMDC 5 4 .035 - .045 Empty .004 - .009 SMDC 5 5 .004 - .009 Empty .212 min SMDC 5 6 .004 -.009 Powder .004 - .009 SMDC 5 7 .004 - .009 Empty .004 - .009 SMDC 5 8 .035 - .045 Powder .150 min LSC 5 9 .035 - .045 Empty .150 min LSC 5 10 .035 - .045 Powder .004 - .009 LSC 5 11 .035 - .045 Empty .004 - .009 LSC 5

Figure 31 shows the test setup for RCDC to LSC propagation. A single terminated end SMDC was torqued into the end-fitting and the un-terminated end was initiated by a #8 blasting cap.

Figure 31. Test Setup for RCDC to LSCA Detonation

Transfer

Figure 32 shows the setup for inverse propagation from LSC to RCDC. The end-fitting was installed with a double end SMDC that was terminated with a “swell cap” for successful propagation verification. The LSC free end was initiated by a #8 blasting cap. In both test configurations, the bottom side of the LSC was taped with a 0.125-in.-thick steel witness plate. Although no standoff was incorporated, it was anticipated that the plate would be severed upon LSC function.

Figure 32. Test Setup for LSCA to RCDC Detonation

Transfer

The results were very successful. All LSCs severed the 0.125-in.-thick steel witness plate. The swell cap expansion ranged from 0.025 to 0.030-in., well above the 0.020-in. minimum requirement and in family with the historical database.

A replacement Lot 7 subscale unit was subjected to lot acceptance test. This unit, along with the original 8 lot acceptance test units, was successfully function tested. These test results verified the acceptability of LSCA Lot 7 as well as the already delivered Lot 5 for flight applications.

SUMMARY

A thorough investigation was performed for a rare bulged closure disc phenomenon observed in the Titan IV Linear Shaped Charge Assembly (LSCA) used in the flight termination system for the Solid Rocket Motor Upgrade strap-on motors, after exposure to the lot acceptance dynamic test environments. The results indicate the bulge was caused by the lot acceptance testing, including the use of subscale test units and the test methodology, both

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of which were not totally representative of full scale flight units.

Theoretical analysis and a thorough testing series both demonstrated that the presence of a bulged disc has no impact on the LSCA function reliability. Parameters evaluated included: the size of the bulge, the maximum and minimum gap between the components across the bulged disc, the presence or absence of broken loose fine explosive powder inside the bulged disc, and the direction of detonation transfer.

ACKNOWLEDGEMENTS

Many Governmental and industrial personnel contributed to the successful work reported herein. Their efforts are gratefully acknowledged: UPCO, D. Lee, D. Way, S. McDonald and the manufacturing and testing crews; ATK Thiokol, Inc., B. Cabe, E. Malloy, J. Robinson and R. Christensen; Lockheed Martin, R. Seybert, B. West, and C. Bunker; The Aerospace Corporation, S. Ben-Shmuel, M. Buechler and S. Goldstein; U.S. Air Force, Eastern Range Safety, S. Phoonkeao and Western Range Safety, J. Nguyen.

REFERENCES

1. Webster, B. D., and Yang, L. C., “Evolution of Ordnance Subsystems and Components in Air Force Strategic Missile Systems,” Vol. 40, No. 4, J. of Spacecraft and Rockets, July-August 2003, pp. 510-522.

2. AIAA 95-2409, “Peacekeeper Flight Termination Ordnance System,” Ramsey, J. A., Sessler, D. R., and Yang, L. C., 31st AIAA/ASME/SAE/ASEE Joint Propulsion Conference, July 10-12, 1995, San Diego, CA.

3. AIAA 2001-3219, “Aging Surveillance Testing of Minuteman II Gas Generator Igniter and Destruct Charge,” Yang, L. C., Dao-Randall, M. T., Pham, D. C., and Estes, T. K., 37th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, July 8-11, 2001, Salt Lake City, UT.

4. “Process Specification, HNS Explosive,” SS345314, Issue B, Sandia National Laboratories, Albuquerque, NM, January 12, 1982.

5. AIAA 99-2420, “Key Parameters for Controlling of Function Reliability in ‘Nonel Tube’ Explosive Transfer System,” Yang, L. C., and Do, P. H., 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, June 20-24, 1999, Los Angeles, CA.

6. Yang, L. C., and Do, I. P. H., “Nonelectrical Tube Explosive Transfer System,” Vol. 38, No. 12, AIAA Journal, December 2000, pp. 2260-2267.

7. AIAA 2000-3734, “Confined Detonating Cord Blowout Analysis,” Yang, L. C., Do, I. P. H., and McMunn, J. C., 36th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, July 16-19, 2000, Huntsville, AL.

8. AIAA 2001-3220, “Titan IVB Linear Shaped Charge Assembly Explosive Train Transfer Reliability,” Yang, L. C. and Do, I. P. H., 37th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, July 8-11, 2001, Salt Lake City, UT.

9. Yang, L. C., and Do, I. P. H., “Titan IVB Linear-Shaped Charge Assembly Explosive Train Transfer Reliability,” Vol. 41, No. 7, AIAA Journal, July 2003, pp. 1304-1313.

10. Range Safety Office, “Airborne Range Safety System Documentation, Design and Test Requirements”, Eastern and Western Range 127-1, Vol. 2, Patrick AFB, FL, Chap.4.

11. Schimmel, M. L., “Quantitative Understanding of Explosive Stimulus Transfer,” NASA CR-2341, December 1973.

12. AIAA 95-2702, “Use of VISAR to Replace Detonator Dent Test,” Neyer, B. T., 31st AIAA/ASME/SAE/ASEE Joint Propulsion Conference, July 10-12, 1995, San Diego, CA.

13. Schwartz, A. C., “A New Technique for Characterization an Explosive for Shock Initiation Sensitivity,” SAND75-0314, Sandia National Laboratories, Albuquerque, NM, December 1975.

14. Gibbs, T. R., and Popolato, A., “LASL Explosive Property Data,” Univ. of California Press, Berkeley, CA, 1980, pp. 48, 299- 301.

15. DOD-E-83578A, “Military Specification, Explosive Ordnance for Space Vehicle (Metric), General Specification for,” USAF, 15 October 1987.

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